Freetards is right
I actually see little wrong with the answers given to the people asking about Firefox at the US State Dept.
"The Vociferous Time Waster" made some excellent points. Firefox is no "freer" than IE, and in a large organization, the costs of deploying any technology are only slightly influenced by what you pay for it in currency. The cost of deploying, managing, supporting, updating and so on BY FAR trump the acquisition costs - many studies have been done on this. It was actually the impetus for coining that ugly term/acronym "TCO" - "Total Cost of Ownership".
Historically I've been not particularly enamored of using FF in a large setting because Mozilla never seemed to give a leap about making it centrally-manageable or easily locked-down. Only recently, after many years, are they making a tiny effort in this regard. I sure as hell don't need users installing a bunch of crappy add-ons whenever they want to, many of which may be designed to circumvent I.T. policies, for example.
Another problem is that FF add-ons are installed on a PER-USER basis, which is a really craptastic thing in an organization where you have to support lots of users.
Lastly - don't for a minute think that FF is automatically "safer" than IE. As FF's marketshare goes up, so do the exploits for it.
Just last week as I was in the middle of installing updates on a laptop for a staff-member, I needed to look up something online, and stupidly (because I was in a hurry and didn't want to go to a different machine) did it with the old version of FF on there. So - did a quick Google search, clicked a couple of links, BLAMMO - machine got hit with a drive-by iFrame exploit, followed by download of a malicious PDF file (exploiting older version of Adobe Reader), and a trojan.
I always prefer to avoid IE whenever possible myself - and sometimes even disable it on user systems I manage - but FF is not the be-all/end-all to all the world's browser problems either.